More Conservation of
Energy
Lesson 11
Announcements
HW #10 due today
 HW #11 (FR #3) due tomorrow
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Lunch Bunch meets today!!!
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Optics repair on Friday
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7am, 4th, 5th, 3pm
AP Physics B Standards
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I.C.3. Conservation of Energy
a) Students should understand the concepts of mechanical energy and
of total energy, so they can:
More Conservation of Energy
LESSON 11:
(1) State and apply the relation between the work performed on an object by
non-conservative forces and the change in an object’s mechanical energy.
(2) Describe and identify situations in which mechanical energy is converted
to other forms of energy.
(3) Analyze situations in which an object’s mechanical energy is changed by
friction or by a specified externally applied force.
Lesson Objectives
Students will be able to
1.
2.
define systems for which energy is conserved
apply principles of energy conservation in modeling real
life situations and performing related calculations.
Work done by nonconservative forces
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Work done by non-conservative forces typically
changes the mechanical energy of a system.
Often, non-conservative work is associated
with a conversion of potential and/or kinetic
energy to thermal energy.
Wnc  E
Sample Problem 11.2:
A 2.3 kg box, starting from rest, is pushed up a ramp by a 10 N force parallel
to the ramp. The ramp is 2.0 m long and tilted at 17°. The speed of the box at
the top of the ramp is 0.80 m/s.
Consider the system to be the box + ramp + Earth.
a) How much work W does the force do on the system?
b) What is the change ∆K in the kinetic energy of the system?
Sample Problem 11.2:
A 2.3 kg box, starting from rest, is pushed up a ramp by a 10 N force parallel
to the ramp. The ramp is 2.0 m long and tilted at 17°. The speed of the box at
the top of the ramp is 0.80 m/s.
Consider the system to be the box + ramp + Earth.
c) What is the change ∆Ug in the gravitational potential energy of the
system?
d) What is the change ∆Eth in the thermal energy of the system?
Sample Problem 11.3:
A 1500 kg car traveling at 20 m/s skids to a halt. What is the
change in thermal energy of the car and the road surface?
Summary of Work
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Work done by conservative forces (Wc)
change the potential energy of a system.
Wc = -U
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Work done by non-conservative forces (Wnc)
change the mechanical energy of a system.
Wnc = E
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Work done by all the forces on a system
(Wnet) changes the kinetic energy of a system.
Wnet = K
Momentum and Energy in
Collisions
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Momentum in collisions is conserved.
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Energy in collisions is conserved.
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Kinetic energy is conserved only for elastic
collisions.
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Sample Problem 12.2:
A 1,000-g cart moving at 2.0 m/s on an air track elastically strikes a
1,000-g cart at rest. What are the resulting velocities of the two carts?
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Sample Problem 12.3:
A 50 g marble moving at 2.0 m/s strikes a 20 g marble at rest. What is
the speed of each marble immediately after the collision?